The electronic structure, linear, and non-linear optical properties of ferroelectric-semiconducto... more The electronic structure, linear, and non-linear optical properties of ferroelectric-semiconductor SbSBr are investigated in the non-polar (paraelectric) and polar (ferroelectric) phase, using the density functional methods in the generalized gradient approximation. The electronic band structure obtained shows that SbSBr has an indirect forbidden gap of 2.16 and 2.21 eV in the paraelectric and ferroelectric phase, respectively. The linear photon-energy dependent dielectric functions and some optical functions, such as absorption and extinction coefficients, refractive index, energy-loss function, reflectivity, and optical conductivity in both phases and photon-energy dependent second-order susceptibilities in the ferroelectric phase are calculated. Moreover, some important optical parameters, such as the effective number of valence electrons and the effective optical dielectric constant, are calculated in both phases.
An ab-initio pseudopotential calculation has been performed by using density functional methods w... more An ab-initio pseudopotential calculation has been performed by using density functional methods within the local density approximation (LDA) to investigate the band structure and optical properties of the ferroelectric-semiconductor SbSI in the para- and ferroelectric phases. It has been shown that SbSI has an indirect gap in both phases (1.45 eV and 1.49 eV in the para- and ferroelectric phases respectively) and that the smallest direct gap is at the S point of the Brillouin zone (1.56 eV and 1.58 eV in the para- and ferroelectric phases respectively). Furthermore, it is shown that first-order phase transition, from the paraelectric phase to the ferroelectric phase (the transiton temperature is about 22 °C), does not change the nature of the band gap. Moreover, the linear frequency dependent dielectric function, including self-energy effects, has been calculated along the c-polar axis in the para- and ferroelectric phases.
In present work, using the density functional theory within local density approximation, the stru... more In present work, using the density functional theory within local density approximation, the structural optimization, electronic band structure, density of electron states, optical, dynamic and thermodynamic properties of HgTe and HgSe were investigated. It was found that HgTe and HgSe show semiconducting property with a direct band gap at high symmetry point gamma with the value of 0.99 eV since conduction band does not interact with uppermost state of valance band because of existence of heavy holes. Real and imaginary parts of dielectric function as a function of photon energy were studied and photon wavelength dependence of refractive index was compared with experimental result. Also, temperature dependent thermodynamic properties such as Helmholtz free energy, internal energy, entropy and specific heat have been worked. The calculated and experimental results are in a good agreement.
ABSTRACT The electronic, linear optic and elastic properties of RbNbO3 crystal have been calculat... more ABSTRACT The electronic, linear optic and elastic properties of RbNbO3 crystal have been calculated under pressure using the density functional theory for its five different phases, P-1, Pm-3m, P4mm, Amm2 and R3m. It is shown that RbNbO3 crystal electrically exhibits a semiconductor property under zero pressure in these phases except for triclinic phase, P-1, in which the compound has insulator property. Also, the crystal has a direct band gap in the Amm2 phase while it has an indirect band gap in the other four phases. Moreover, the electronic properties were calculated under pressure and it is shown that the locations of top of the valence band and the bottom of the conduction band in the Brillouin Zone were varied with the effect of pressure. The linear optical functions such as dielectric function, refractive index, absorption and extinction coefficients depend on light frequency were calculated under various pressure values for all phases and it is shown that values of the optical constants change with the effect of pressure. Finally, the elastic stiffness tensor of RbNbO3 were calculated for four phases, Pm-3m, P4mm, Amm2 and R3m and the variations of some elastic constants, such as Debye temperature, Poisson ratio, bulk modulus, shear modulus, and Young modulus, with the pressure were investigated.
On the basis of first principles calculations using density functional theory, we explore the str... more On the basis of first principles calculations using density functional theory, we explore the struc- tural and electronic properties of two binaries: CaO and MgO in rock salt structures. Structural properties of the semiconductor x 1 Ca Mg O − x alloys are derived from total-energy minimization within the General Gradient Approximation. The band gap bowing parameters dependence is very powerful Calcium composition. The results offer that an average bowing parameter of x 1 Ca Mg O − x alloys is b = ~0.583$ eV. We analyzed the volume deformation, charge transfer and structural relaxation effects of the x 1 Ca Mg O − x alloys.
In this study, the geometric structural optimization, electronic band structure and total density... more In this study, the geometric structural optimization, electronic band structure and total density of states for electrons of ferroelectric Bi 2 NbO 5 F structure with space group Pca2 1 at room temperature have been investigated by Augmented Plane Wave method (APW) using the density functional theory (DFT) under the local density approximation (LDA). The ground state properties of ferroelectric Bi 2 NbO 5 F structure are studied. The computed ground state properties and experimental results are consistent. The energy band structure of Bi 2 NbO 5 F compound below T c shows that it is a ferroelectric semiconductor with narrow band gap of 0.0589 eV. Unfortunately, there is neither experimental nor theoretical electronic band structure study of Bi 2 NbO 5 F crystal in the literature, so we could not compare our results.
Abstract: The definitions of all symmetry operations, their permutation counterparts and matrix r... more Abstract: The definitions of all symmetry operations, their permutation counterparts and matrix representations, the multiplication table, all classes, abelian subgroups, and some other subgroups of point group Oh are clearly obtained. For every symmetry operation (or element) of Oh, appropriate figures are pictured for the sake of clarity.
The structural, electronic and optical properties of KCdF3 are investigated using the density fun... more The structural, electronic and optical properties of KCdF3 are investigated using the density functional theory (DFT) within the generalized gradient approximation (GGA). The calculated lattice parameters have been compared to experimental results and demonstrated to be in good agreement with them. The calculated electronic band structure of cubic KCdF3 shows that crystal has a indirect forbidden band gap with value of 2.95 eV from the high symmetry point R to gamma point in the first Brillouin Zone (BZ). The optical spectra are investigated under the scissor approximation in the photon energy range, up to 30 eV. The dielectric function and some optical constants such as energy loss functions, reflectivity, extinction, and absorption coefficients, effective number of valance electrons and refractive index are calculated.
We introduce a method for calculating the amount of deflection angle of light passing close to a ... more We introduce a method for calculating the amount of deflection angle of light passing close to a massive object. It is based on Fermat’s principle. The varying refractive index of medium around the massive object is obtained from the Buckingham pi-theorem.
Euroasia Journal of Mathematics, Engineering, Natural & Medical Sciences
In this study, the point groups 𝐷2𝑑 and 𝐶3𝑖 which belong to tetragonal and trigonal crystal syste... more In this study, the point groups 𝐷2𝑑 and 𝐶3𝑖 which belong to tetragonal and trigonal crystal systems, respectively, are handled under the class sum approach. Symmetry groups were formed with symmetry elements that left these point groups unchanged and Cayley tables of related groups were obtained. Using these tables, the conjugates of the elements and the classes of the group were formed. Secular equations are written for each class sum obtained by the sum of the elements that make up the class. By solving these secular equations, the character vectors are obtained. Thus, the character tables were reconstructed with the calculated characters for both point groups under the class sum approach.
International Journal for Simulation and Multidisciplinary Design Optimization
The geometric structural optimization, electronic band structure, total density of states for val... more The geometric structural optimization, electronic band structure, total density of states for valence electrons, density of states for phonons, optical, dynamical, and thermodynamical features of cesium chloride have been investigated by linearized augmented plane wave method using the density functional theory under the generalized gradient approximation. Ground state properties of cesium chloride are studied. The calculated ground state properties are consistent with experimental results. Calculated band structure indicates that the cesium chloride structure has an indirect band gap value of 5.46 eV and is an insulator. From the obtained phonon spectra, the cesium chloride structure is dynamically stable along the various directions in the Brillouin zone. Temperature dependent thermodynamic properties are studied using the harmonic approximation model.
The electronic energy band structure and linear optical properties of the ferroelectric semicondu... more The electronic energy band structure and linear optical properties of the ferroelectric semiconductor SbSI in the paraelectric phase are calculated by an ab initio pseudopotential method using density functional theory in the local density approximation. The calculated electronic band structure shows that SbSI has an indirect band gap of 1.45 eV and that the smallest direct gap is at the
The electronic structure, linear, and non-linear optical properties of ferroelectric-semiconducto... more The electronic structure, linear, and non-linear optical properties of ferroelectric-semiconductor SbSBr are investigated in the non-polar (paraelectric) and polar (ferroelectric) phase, using the density functional methods in the generalized gradient approximation. The electronic band structure obtained shows that SbSBr has an indirect forbidden gap of 2.16 and 2.21 eV in the paraelectric and ferroelectric phase, respectively. The linear photon-energy dependent dielectric functions and some optical functions, such as absorption and extinction coefficients, refractive index, energy-loss function, reflectivity, and optical conductivity in both phases and photon-energy dependent second-order susceptibilities in the ferroelectric phase are calculated. Moreover, some important optical parameters, such as the effective number of valence electrons and the effective optical dielectric constant, are calculated in both phases.
An ab-initio pseudopotential calculation has been performed by using density functional methods w... more An ab-initio pseudopotential calculation has been performed by using density functional methods within the local density approximation (LDA) to investigate the band structure and optical properties of the ferroelectric-semiconductor SbSI in the para- and ferroelectric phases. It has been shown that SbSI has an indirect gap in both phases (1.45 eV and 1.49 eV in the para- and ferroelectric phases respectively) and that the smallest direct gap is at the S point of the Brillouin zone (1.56 eV and 1.58 eV in the para- and ferroelectric phases respectively). Furthermore, it is shown that first-order phase transition, from the paraelectric phase to the ferroelectric phase (the transiton temperature is about 22 °C), does not change the nature of the band gap. Moreover, the linear frequency dependent dielectric function, including self-energy effects, has been calculated along the c-polar axis in the para- and ferroelectric phases.
In present work, using the density functional theory within local density approximation, the stru... more In present work, using the density functional theory within local density approximation, the structural optimization, electronic band structure, density of electron states, optical, dynamic and thermodynamic properties of HgTe and HgSe were investigated. It was found that HgTe and HgSe show semiconducting property with a direct band gap at high symmetry point gamma with the value of 0.99 eV since conduction band does not interact with uppermost state of valance band because of existence of heavy holes. Real and imaginary parts of dielectric function as a function of photon energy were studied and photon wavelength dependence of refractive index was compared with experimental result. Also, temperature dependent thermodynamic properties such as Helmholtz free energy, internal energy, entropy and specific heat have been worked. The calculated and experimental results are in a good agreement.
ABSTRACT The electronic, linear optic and elastic properties of RbNbO3 crystal have been calculat... more ABSTRACT The electronic, linear optic and elastic properties of RbNbO3 crystal have been calculated under pressure using the density functional theory for its five different phases, P-1, Pm-3m, P4mm, Amm2 and R3m. It is shown that RbNbO3 crystal electrically exhibits a semiconductor property under zero pressure in these phases except for triclinic phase, P-1, in which the compound has insulator property. Also, the crystal has a direct band gap in the Amm2 phase while it has an indirect band gap in the other four phases. Moreover, the electronic properties were calculated under pressure and it is shown that the locations of top of the valence band and the bottom of the conduction band in the Brillouin Zone were varied with the effect of pressure. The linear optical functions such as dielectric function, refractive index, absorption and extinction coefficients depend on light frequency were calculated under various pressure values for all phases and it is shown that values of the optical constants change with the effect of pressure. Finally, the elastic stiffness tensor of RbNbO3 were calculated for four phases, Pm-3m, P4mm, Amm2 and R3m and the variations of some elastic constants, such as Debye temperature, Poisson ratio, bulk modulus, shear modulus, and Young modulus, with the pressure were investigated.
On the basis of first principles calculations using density functional theory, we explore the str... more On the basis of first principles calculations using density functional theory, we explore the struc- tural and electronic properties of two binaries: CaO and MgO in rock salt structures. Structural properties of the semiconductor x 1 Ca Mg O − x alloys are derived from total-energy minimization within the General Gradient Approximation. The band gap bowing parameters dependence is very powerful Calcium composition. The results offer that an average bowing parameter of x 1 Ca Mg O − x alloys is b = ~0.583$ eV. We analyzed the volume deformation, charge transfer and structural relaxation effects of the x 1 Ca Mg O − x alloys.
In this study, the geometric structural optimization, electronic band structure and total density... more In this study, the geometric structural optimization, electronic band structure and total density of states for electrons of ferroelectric Bi 2 NbO 5 F structure with space group Pca2 1 at room temperature have been investigated by Augmented Plane Wave method (APW) using the density functional theory (DFT) under the local density approximation (LDA). The ground state properties of ferroelectric Bi 2 NbO 5 F structure are studied. The computed ground state properties and experimental results are consistent. The energy band structure of Bi 2 NbO 5 F compound below T c shows that it is a ferroelectric semiconductor with narrow band gap of 0.0589 eV. Unfortunately, there is neither experimental nor theoretical electronic band structure study of Bi 2 NbO 5 F crystal in the literature, so we could not compare our results.
Abstract: The definitions of all symmetry operations, their permutation counterparts and matrix r... more Abstract: The definitions of all symmetry operations, their permutation counterparts and matrix representations, the multiplication table, all classes, abelian subgroups, and some other subgroups of point group Oh are clearly obtained. For every symmetry operation (or element) of Oh, appropriate figures are pictured for the sake of clarity.
The structural, electronic and optical properties of KCdF3 are investigated using the density fun... more The structural, electronic and optical properties of KCdF3 are investigated using the density functional theory (DFT) within the generalized gradient approximation (GGA). The calculated lattice parameters have been compared to experimental results and demonstrated to be in good agreement with them. The calculated electronic band structure of cubic KCdF3 shows that crystal has a indirect forbidden band gap with value of 2.95 eV from the high symmetry point R to gamma point in the first Brillouin Zone (BZ). The optical spectra are investigated under the scissor approximation in the photon energy range, up to 30 eV. The dielectric function and some optical constants such as energy loss functions, reflectivity, extinction, and absorption coefficients, effective number of valance electrons and refractive index are calculated.
We introduce a method for calculating the amount of deflection angle of light passing close to a ... more We introduce a method for calculating the amount of deflection angle of light passing close to a massive object. It is based on Fermat’s principle. The varying refractive index of medium around the massive object is obtained from the Buckingham pi-theorem.
Euroasia Journal of Mathematics, Engineering, Natural & Medical Sciences
In this study, the point groups 𝐷2𝑑 and 𝐶3𝑖 which belong to tetragonal and trigonal crystal syste... more In this study, the point groups 𝐷2𝑑 and 𝐶3𝑖 which belong to tetragonal and trigonal crystal systems, respectively, are handled under the class sum approach. Symmetry groups were formed with symmetry elements that left these point groups unchanged and Cayley tables of related groups were obtained. Using these tables, the conjugates of the elements and the classes of the group were formed. Secular equations are written for each class sum obtained by the sum of the elements that make up the class. By solving these secular equations, the character vectors are obtained. Thus, the character tables were reconstructed with the calculated characters for both point groups under the class sum approach.
International Journal for Simulation and Multidisciplinary Design Optimization
The geometric structural optimization, electronic band structure, total density of states for val... more The geometric structural optimization, electronic band structure, total density of states for valence electrons, density of states for phonons, optical, dynamical, and thermodynamical features of cesium chloride have been investigated by linearized augmented plane wave method using the density functional theory under the generalized gradient approximation. Ground state properties of cesium chloride are studied. The calculated ground state properties are consistent with experimental results. Calculated band structure indicates that the cesium chloride structure has an indirect band gap value of 5.46 eV and is an insulator. From the obtained phonon spectra, the cesium chloride structure is dynamically stable along the various directions in the Brillouin zone. Temperature dependent thermodynamic properties are studied using the harmonic approximation model.
The electronic energy band structure and linear optical properties of the ferroelectric semicondu... more The electronic energy band structure and linear optical properties of the ferroelectric semiconductor SbSI in the paraelectric phase are calculated by an ab initio pseudopotential method using density functional theory in the local density approximation. The calculated electronic band structure shows that SbSI has an indirect band gap of 1.45 eV and that the smallest direct gap is at the
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Papers by Harun Akkus